The delivery target for Airmail outside of Europe is 5 days. I can't recall posting anything to South Africa but my experience of posting stuff to places like Australia and the USA is that the target is usually met. (Indeed, it's often beaten). So if you post a card tomorrow it should arrive with 4 days to spare before the 10th.
Propably OK if it`s going to Cape Town as it only has to go on an aircraft at this end and off an aircraft at that end. If it was elsewhere in SA it would be quite a bit longer. My sister spends a lot of time out there and postcards can be hit and miss. Probably quicker posting from here than the other way around.